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In the tv series Star Trek they refer to space as "the final frontier". Now, that series was set way in the future, but today, with just over 500 humans having gone into the microgravity of space, we have already learnt that space is really the hostile frontier.

On one hand, the environment inside the International Space Station as it zips around the Earth once every one-and-a-half hours seems perfectly benign. After all, the astronauts are wearing casual clothing such as shorts and t-shirts.

But strange things happen in space.

Consider the astronaut Richard Hieb. At six foot, three inches tall he was just one inch under the official NASA height limit for astronauts of six foot, four inches. On 8 June 1994 he went into orbit on the Space Shuttle Columbia. Within four days he had grown more than an inch in height. And so, was technically too tall to fly on the space shuttle. You see, while he was on Earth gravity compressed the discs in his spine, but in the microgravity of space that compression was removed. Astronauts have grown up to two inches — or 50 millimetres — in height. That extra height can stretch their sciatic nerve and cause excruciating back pain.

Another problem area is the astronaut's fluid circulation. Earth's gravity tends to pull the fluids in our body downward. But in the microgravity of space the various fluids rise. So astronauts' legs shrink, but their faces swell — they can even get headaches. Surprisingly because they get fluids accumulating in their nose astronauts say their sense of smell is weakened, and as a direct result, food tastes bland.

Down on Earth, the mere act of standing and walking puts stresses on the bone. And the bone responds by rebuilding itself to oppose those stresses. But in the microgravity of space you lose that continual bone stress so the bone loses minerals and density.

There are two results from the loss of bone minerals.

First, the bones weaken enormously, putting the astronaut at a risk of fractures upon returning to Earth. For example, down here on Earth post-menopausal women can lose one to one-and-a-half per cent of their bone mass in the hip in one year. But an astronaut in microgravity can lose that much in a single month. And yes, when they do return to Earth they can recover most of their bone loss. But the way their bone is laid down is not as strong as before their space journey and they will have a greater risk of bone fractures for the rest of their life.

The second problem with bone loss in space is that the minerals they lose from their bones have to go somewhere. They find their way to the kidneys, and once there can cause the very painful condition of kidney stones.

Another problem of microgravity is that the heart changes shape and becomes more spherical. It also functions less efficiently. Astronauts can lose one quarter of their aerobic capability after as little as two weeks in orbit. Some astronauts have suffered abnormalities in their heart rhythms during space flight and others have fainted when they've returned to Earth because their blood pressure fell so low.

Some 60 per cent of astronauts were found to undergo eye damage as a result of microgravity — sometimes, permanently. Remember how the bodily fluids shifted upwards from the legs? The increased pressure inside the skull forced the backs of their eyeballs to become less round and more flat. As a result, the image came to a focus behind the retina and gave them blurry vision.

Another problem area is muscle. Muscles are made up of muscle fibres. Down here on Earth the activities of daily living keep your muscle fibres in good condition, but spaceflights of even less than 14 days were enough to make muscle fibres shrink by one-third. Longer missions could make them shrink by as much as 40 per cent.

Radiation is another hazard. Now there are different ways of measuring radiation but one is the millisievert. The average Australian gets a natural background radiation dose of about one-and-a-half millisieverts per year — equivalent of about five chest x-rays. But in space, astronauts can be exposed to 2000 millisieverts.

Another problem is that the functioning of the immune system is altered in space — and for the worse. For example, many of us have herpes which our immune system keeps at bay. But astronauts have been repeatedly observed to have their latent herpes viruses reactivated even during short-duration spaceflights.

Now we humans will become a space-going race. At some stage more humans will be living off the planet than on it. Earth has been our cradle and now we are learning how to grow up and leave home. And growing pains are all part of that ...